The present invention relates to an improved process for the Fischer-Tropsch reaction, which essentially consists in a first reaction phase in a gas-liquid-solid fluidized reactor and a second separation phase, at least partial, internal or external, of the solid suspension in the liquid.
The Fischer-Tropsch reaction consists in the production of essentially linear and saturated hydrocarbons, preferably having at least 5 carbon atoms in the molecule, by means of the catalytic hydrogenation of CO, optionally diluted with CO2.
The reaction between CO and H2 is carried out in a gas-liquid-solid fluidized reactor in which the solid, prevalently consisting of particles of catalyst, is suspended by means of the gas stream and liquid stream. The former prevalently consists of reagent species, i.e. CO and H2, whereas the latter consists of hydrocarbons produced by the Fischer-Tropsch reaction, optionally at least partially recycled, either from the material liquid under the process conditions, or the relative mixtures.
The gas and liquid, optionally recycled, are fed from the bottom of the column by means of appropriate distributors and the flow-rates of the gas and liquid are such as to guarantee a turbulent flow regime in the column.
In gas-liquid-solid fluidized systems such as that of the Fischer-Tropsch reaction, the flow-rates of the fluids should be such as to guarantee a practically homogeneous suspension of the solid in the whole reaction volume and facilitate the removal of the heat produced by the exothermic reaction, improving the heat exchange between the reaction zone and a suitable exchanger device inserted in the column.
In addition, the solid particles should have dimensions which are sufficiently large as to enable them to be easily separated from the liquid products, but sufficiently small as to render the diffusive intra-particle limitations negligible (unitary particle efficiency) and enable them to be easily fluidized.
The average diameter of the solid particles used in slurry reactors can vary from 1 to 200 xcexcm, although operating with dimensions of less than 10 xcexcm makes the separation of the solid from the liquid products extremely expensive.
In the Fischer-Tropsch process, as in all three-phase processes in the presence of catalysts, there is therefore the problem of an optimum particle dimension in both the reaction and separation steps.
As far as the fluidization of the solid particles is concerned, EP-A-450,860 discloses operating in reaction phase with a slurry bubble column under optimum conditions when the following equation is respected:
0.5 (Usxe2x88x92Ul)xe2x89xa6D/Hxe2x80x83xe2x80x83(1)
wherein Ul is the circulation velocity of the liquid phase, D is the axial dispersion coefficient of the solid phase, H is the dispersion height (gas+liquid+solid) and Us is the settling velocity of the particles defined as follows:                               U          s                =                                            1              18                        ·                          d              p              2                                ⁢                                                                      ρ                  s                                -                                  ρ                  l                                                            μ                L                                      ·            g            ·                          f              ⁡                              (                                  C                  p                                )                                                                        (        2        )            
wherein dp is the average particle diameter, xcfx81s is the density of the solid, xcfx81l is the density of the liquid, xcexc the viscosity of the liquid, g the gravity acceleration and f(Cp) represents the hindering function due to the presence of other particles and depending on the volumetric concentration of the particles Cp.
The description of EP""860, however, is very incomplete and discloses, moreover, the use of particles with very small dimensions, with obvious limits in the solid-liquid separation step. In other words, the technical problem of EP""860 relates only to the reaction phase and not to the whole process, comprising both the reaction and solid-liquid separation.
Above all, EP""860 does not indicate any method or correlation for determining the axial dispersion coefficient of the solid, D (a fundamental parameter in verifying the constraint (1)), neither does it provide any experimental values of D for comparison. In addition, if one succeeds in obtaining a value of D, assuming a dispersion height H=2D/(Usxe2x88x92Ul) (a value which is at the limit of the validity range of (1)), the concentration of the solid proves to decrease from the bottom to the top of the reaction volume by a factor of 7.4. If this height is halved, the reduction factor of the concentration of the solid decreases to 2.4 which however is very high. As mentioned above, on the other hand, an optimum condition for a slurry reactor should comprise a uniform concentration profile in the whole catalyst volume.
EP-A-450,860 also discloses operating according to Stokes"" law: it is in fact known in literature that the term,                     1        18            ·              d        p        2              ⁢                                        ρ            s                    -                      ρ            l                                    μ          L                    ·      g        ,
introduced in the definition of Us of equation (2), represents the terminal settling velocity of the particle, Ut, according to Stokes"" law. This law (see Perry""s Chemical Engineers"" Handbook, 6th Ed.) is valid in the laminar regime when the Reynolds"" particle number Rep is less than 0.1. As the Reynolds"" number is a function of the properties of the liquid-solid system and of the particle dimensions, once the liquid phase (Fischer-Tropsch synthesis waxes) and type of solid (catalyst for Fischer-Tropsch synthesis, for example Cobalt supported on alumina) have been established, there is a higher limit for the average particle diameter, over which Stokes"" law is no longer valid.
As a result EP""860 discloses operating with particle dimensions of over 5 xcexcm, but not exceeding the limit value of dp established by Stokes"" law.
For example, considering the data provided in EP""860 for a system consisting of Fischer-Tropsch waxes and Cobalt supported on Titania (xcfx81l=0.7 g/cm3, xcfx81s=2.7 g/cm3, xcexc=1 cP), for Stokes"" law to be valid, i.e. Rep less than 0.1, the average particle diameter must be less than 51 xcexcm (see example 1 of EP""860 for further details).
As is well known to experts in the field, this particle diameter, although excellent for the bubble column in reaction phase, creates drawbacks in the catalyst/liquid separation phase.
A method has now been found for effecting the Fischer-Tropsch process which overcomes the above disadvantages as it allows an optimized operation both in the reaction phase and in the solid-liquid separation phase, without substantially varying the activity of the catalyst.
In accordance with this, the present invention relates to an optimized method for the production of heavy hydrocarbons according to the Fischer-Tropsch process and the relative separation of the above hydrocarbons, starting from mixtures of reagent gases, essentially consisting of CO and H2, optionally diluted with CO2, in the presence of supported catalysts, which comprises:
(a) feeding the reagent gases into a reactor, preferably from the bottom, so as to obtain a good dispersion of the solid in the liquid phase, in this way at least partially transforming the reagent gases into heavy hydrocarbons, the gas flow-rates being such as to operate under heterogeneous or churn-turbulent flow conditions (i.e. in the presence of a wide size distribution of the bubbles of gas in the column, normally from about 3 mm to about 80 mm);
(b) at least partially recovering the heavy hydrocarbons formed in step (a) by their external or internal separation from the catalytic particles;
the above process being characterized in that in step (a) the reaction takes place:
(1) in the presence of solid particles so that the particle Reynolds"" number (Rep) is greater than 0.1, preferably from 0.11 to 50, even more preferably from 0.2 to 25, wherein       Re    p    =                    d        p            ·      ν      ·              ρ        l              μ  
wherein dp is the average particle diameter, v is the relative velocity between particle and liquid, xcfx81l is the density of the liquid, xcexc is the viscosity of the liquid;
(2) maintaining the solid particles suspended at a height H, with such Us, Ul and Ug values as to have a Bodenstein number Bosxe2x89xa61, preferablyxe2x89xa60.4.
The Bodenstein number (Bos) is defined as Bos=Pes (Usxe2x88x92Ul)/Ug, wherein Pes is the Peclet number of the solid, Us is the sedimentation rate of the solid, Ul is the circulation velocity of the liquid phase, Ug is the superficial gas velocity. The Peclet number of the solid (Pes) is defined as Pes=Ug.H/Dax,s, wherein H is the height of the dispersion (liquid+solid+gas) and Dax,s, is the axial dispersion coefficient of the solid phase.
The catalysts used in the process of the present invention generally comprise metals of Group VIII, such as Iron, Cobalt, Ruthenium or relative mixtures on carriers of inorganic oxides. The above catalysts may contain additional promoters comprising metals selected from those of Group I, Group II, Group V, Group VII, alone or in mixtures.
The preferred catalysts which can be used in the process of the present invention comprise cobalt, optionally containing promoters, supported on inorganic oxides of at least one of the elements selected from Si, Ti, Al, Zn, Sn, Mg, Th. As far as the surface area of the carrier is concerned, this is within the range of 20-300 m2/g, preferably 50-200 m2/g (BET).
When promoters are contained, these are present in such a quantity as to have a weight ratio between promoter and cobalt of 0.01/1 to 1/1, preferably from 0.025/1 to 0.1/1. When the catalyst contains cobalt, it is present in a quantity ranging from 2 to 50% by weight, preferably from 5 to 20% by weight.
The catalysts which can be used in the process of the present invention can be prepared with the known techniques, for examples by means of gelation, cogelation, impregnation, precipitation, dry impregnation, co-precipitation or mechanical mixing. In the preferred embodiment, the cobalt and optional promoters are linked to the carrier by putting the carrier itself in contact with a solution of a compound containing cobalt (or other possible promoters) by means of impregnation. Optionally the cobalt and possible promoters can be co-impregnated on the carrier itself. The compounds of Cobalt and optional promoters used in the impregnation can consist of any organic or inorganic metal compound susceptible to decomposing after heating in nitrogen, argon, helium or another inert gas, calcination in a gas containing oxygen, or treatment with hydrogen, at high temperatures, to give the corresponding metal, metal oxide, or mixtures of the metal or metal oxide phases.
Compounds of Cobalt (and possible promoters) can be used, such as nitrate, acetate, acetylacetonate, carbonyl naphthenate and the like. The quantity of impregnation solution should be sufficient to completely wet the carrier, usually within a range of about 1 to 20 times the carrier by volume, depending on the concentration of metal (or metals) in the impregnation solution.
The impregnation treatment can be carried out within a wide range of temperature conditions. After impregnation, the catalyst is dried by heating to a temperature of over 30xc2x0 C., preferably from 30xc2x0 C. to 125xc2x0 C., in the presence of nitrogen or oxygen, or both or air, in a gas stream or under partial vacuum. The catalyst particle distribution is obtained within the desired dimensional range by the use of preformed carriers or with the usual techniques such as crushing, ultrasonic treatment or other procedures. Finally, the catalyst particles, are treated to obtain the desired dimensions using known techniques such as, for example, sieving.
The liquid phase necessary for fluidizing the catalyst can be any substance liquid under the reaction pressure and temperature conditions, capable of maintaining the catalyst under suspension, relatively inert under the reaction conditions, and of being a good solvent for carbon monoxide and hydrogen. Typical examples of organic liquids which can be used in the present process are paraffins, olefins, aromatic hydrocarbons, ethers, amines and relative mixtures, provided they are high-boiling. High-boiling paraffins comprise C10-C50 linear or branched paraffins; high-boiling olefins comprise liquid polyalpha-olefins; high-boiling aromatic hydrocarbons comprise single, multiple or condensed ring aromatic hydrocarbons. The preferred liquid hydrocarbon solvent is octacosane or hexadecane; n-paraffinic wax, i.e. the Fischer-Tropsch reaction product, is even more preferable.
The reaction conditions for the Fischer-Tropsch process are generally known to experts in the field. The temperature normally ranges from 160xc2x0 C. to 360xc2x0 C., preferably from 190xc2x0 C. to 230xc2x0 C., even more preferably from 190 to 220xc2x0 C. The pressures are usually higher than 6 bars, preferably from 6 to 60 bars, more preferably from 10 to 30 bars. With an increase in temperature, the conversion of CO and selectivity to methane generally increase, whereas the stability of the catalyst decreases. Consequently, with an increase in the CO conversion due to the temperature, the yield to the desired products, i.e. C5+, preferably C10+, may not increase.
The ratios between carbon monoxide and hydrogen can vary within a wide range. Although the stoichiometric ratio H2/CO in the Fischer-Tropsch process is 2.1/1, in most cases a lower H2/CO ratio is used. For example, U.S. Pat. No. 4,681,867 describes preferred H2/CO ratios ranging from 1/2 to 1/1.4. In any case the process of the present invention is not limited to low H2/CO ratios. In fact, H2/CO ratios ranging from about 1.5/1 to about 2.5/1, preferably from about 1.2/1 to about 2.2/1, can be used.
In the reaction zone of the present invention, the catalyst is suspended and mixed prevalently by the movement induced by the bubbles of gas which rise along the column.
The present invention refers to a gas-liquid-solid system in which the gas flow-rate is such as to have a turbulent flow regime, characterized by a wide distribution of the bubble diameters (3-80 mm approx.) which rise through the column. The mixing and distribution of the catalyst inside the bubble column reactor prevalently derives from the fraction of gas which runs through the column in the form of large bubbles (about 20-80 mm), and drags in its upward motion, at a rising rate of the large bubbles in the order of 1-2 m/s approximately, both the liquid and solid suspended in the liquid. The gas therefore causes macro-vortexes of the continuous phase (liquid) in which the solid is suspended, increasing the dispersion degree of the solid and consequently the uniformity of the axial concentration profile of the solid, with respect to operating in a homogeneous flow regime (low gas flow-rates, gas bubbles uniformly distributed and with small dimensions, 3-6 mm).
We would like to point out that the process of the present invention comprises operating, in the reaction step (a), with a Reynolds"" number of the catalytic particle Rep greater than 0.1, preferably from 0.11 to 50.
As will be explained further on in the examples, the Reynolds"" number (Rep) is a function of the density and viscosity of the liquid phase and also of the density of the catalyst particle and its dimensions. When waxes of the Fischer-Tropsch process are used as reaction liquid (therefore establishing the properties of the liquid phase under the reaction conditions), the Reynolds"" number may only vary in relation to the density and dimensions of the catalytic particles. The expert in the field who knows the density of the catalytic particles he intends to use (normally similar to the density of the inert carrier material), can obtain the average diameter of particles which are such as to have a Reynolds"" number greater than 0.1, preferably from 0.11 to 50, even more preferably from 0.2 to 25.
As far as the effect of the particle diameter on the catalyst activity is concerned, it is known from literature (Iglesia et al., Computer Aided Design of Catalysts, Ed. Becker-Pereira, 1993) that, for supported cobalt based catalysts for the Fischer-Tropsch synthesis, when operating with particles having dimensions of less than 200 xcexcm, there are no substantial reductions in the catalytic performances due to intra-particle diffusion phenomena.
Step (b) of the process of the present invention comprises recovering, at least partially, the liquid products generated by the Fischer-Tropsch reaction by means of extraction from the reaction zone of a certain amount of slurry (liquid+solid). The separation of the desired quantity of liquid products is effected using equipment such as for example hydrocyclones or filters (tangential or frontal) or, preferably, static decanters. The separation step also generates a more concentrated slurry which can be recycled directly to the Fischer-Tropsch reactor, or it can be treated in a regeneration step of the catalyst or it can be partially removed to introduce fresh catalyst. The whole extraction process of the slurry for the separation of the liquid products and reintegration of the more concentrated slurry, partially regenerated and/or substituted, is regulated so as to keep the reaction volume and average concentration of the catalyst constant.
In the case of liquid-solid separation inside the reaction zone, it is possible to use filtration devices (for example cartridge filters) completely immersed in the slurry (liquid+solid) under reaction. When operating under turbulent flow regime conditions, the high rate of the phases (gas, liquid, solid) that lap against the filters, prevents or minimizes the formation of the solid panel, thus reducing interventions for maintenance and regeneration of the filtrating surface.
It should be pointed out that step (b) of the process of the present invention is carried out under favourable conditions. It is known, in fact, that for a certain flow-rate of slurry (liquid+solid), with an increase in the particle diameter, not only are the volumes of the separation section reduced, but the type of equipment necessary for separating the liquid products from the concentrated slurry is simplified. When particles having an average diameter of 150 xcexcm rather than 5 xcexcm are adopted, and with the use of hydrocyclones as separation devices, the unit number is drastically reduced; at the same time the dimensions of the single unit can be increased, thus facilitating the construction of the hydrocyclones themselves (see example 8 for further details) For particles having an average diameter higher than 100-150 xcexcm it is possible to substitute the hydrocyclones with static separators (decanters), making the separation step easier and less expensive.
The process of the present invention is characterized in that it is effected not only within a certain Reynolds"" number range, but also under such conditions as to have a reasonably uniform concentration profile of the solid, Cp (x), along the reaction column; for example a profile Cp (x) which varies by a maximum value of xc2x120% with respect to the average concentration value of the solid (catalyst), {overscore (C)}p. This is equivalent to having a Bodenstein number (Bos) less than or equal to 0.4.
The concentration profile of the solid with respect to the axial co-ordinate of the bubble column reactor, is thus expressed as a function of the Bodenstein number, Bos, which among other parameters, is a function of the diameter of the column. As the diameter of the column increases, maintaining the other parameters constant, the mixing degree of the solid increases, thus improving the distribution of the catalyst inside the reactor. On the basis of the correlations indicated in the following examples, it is possible to determine the minimum diameter of the column sufficient to respect the constraint set for obtaining an optimum distribution of the solid. The value of this diameter is also a function of the solid particle dimensions. With an increase in the average diameter of the particles, the minimum diameter of the column increases: it is therefore possible to obtain an excellent dispersion of the solid phase by suitably dimensioning the reactor.